Transistor and CVD apparatus used to deposit gate insulating film thereof

ABSTRACT

In a transistor adapted to suppress characteristic degradation resulting from fluorine contained in a deposited film, the concentration of fluorine contained in a gate insulating film is reduced to 1.0×10 20  atoms/cm 3  or less. As a result, the transistor can provide excellent reliability even when it is continuously driven for a long period of time at a relatively high temperature.

FIELD OF THE INVENTION

The present invention relates to an improvement in a transistor and,more particularly, to a method for improving the reliability thereofwhen the transistor is continuously driven for a long period of time ata relatively high temperature.

DESCRIPTION OF THE RELATED ART

Among field-effect transistors each of which typically comprises atleast a source electrode, a drain electrode, a gate electrode, asemiconductor film, and a gate insulating film each interposed betweenthe source/drain electrodes and the gate electrode, a field-effecttransistor comprising an amorphous silicon nitride film deposited by aCVD (Chemical Vapor Deposition) method or the like as the gateinsulating film has excellent ON/OFF-state current characteristics sothat, in recent years, it has been used also as a switching element fora liquid crystal display device or the like.

When a thin film such as a gate insulating film is deposited by using aCVD apparatus, cleaning of the inside of the reaction chamber of the CVDapparatus is performed by using a cleaning gas such as NF₃, CF₄, or SF₆in each given deposition cycle to improve the maintenance property ofthe CVD apparatus and thereby improve the operability thereof. At thistime, if fluorine as one component of the cleaning gas remains in thereaction chamber and is caught in the film being deposited, it causesthe problem of significantly degrading the transistor characteristics.

As a method for solving this problem, the reduction of the concentrationof fluorine contained in the semiconductor film to 1.0×10¹⁹ atoms/cm³ orless is disclosed in Patent Gazette 1. As means for reducing theconcentration of fluorine, the removal of residual fluorine bygenerating a hydrogen plasma after the cleaning of the reaction chamberis disclosed. It is reported that, by reducing the concentration offluorine contained in the semiconductor film, an increase in the amountof shift in threshold voltage when the transistor is operated for 10minutes under the condition that the substrate temperature is 25.0±3.0°C. can be reduced and the reliability of the transistor can be therebyimproved.

As an example of an approach which reduces the concentration of fluorinein a thin film other than a thin film composing the transistor such asthe semiconductor film or the gate insulating film, a method isdisclosed in Patent Gazette 2. In accordance with the method, when asilicon dioxide film is disposed as a protective film for an insulatingsubstrate between the transistor and the insulating substrate, theconcentration of fluorine in the silicon dioxide film is reduced to1×10¹⁹ atoms/cm³ or less, preferably to 1×10¹⁸ atoms/cm³ or less, byspraying heated gas to an a-Si film deposited by a plasma CVD method andthereby oxidizing the a-Si film into the silicon dioxide film.

Patent Gazette 1: Japanese Laid-Open Patent Publication No. 2002-329869(page 2, FIG. 1)

Patent Gazette 2: Japanese Laid-Open Patent Publication No. 2003-124469(page 2, FIG. 1)

SUMMARY OF THE INVENTION

In the method disclosed in Patent Gazette 1, however, evaluation isperformed at a relatively low temperature of about 25° C. duringshort-period driving that continues for only 10 minutes. Therefore, itis unknown whether or not the transistor can provide excellentreliability even when it is continuously driven for a long period oftime (on the orders of several to several tens of hours) at a relativelyhigh temperature (e.g., about 80° C.), like a transistor as a switchingelement for a pixel electrode in a liquid crystal display device.

In the method which removes the residual fluorine in the reactionchamber therefrom by the hydrogen plasma process, a sufficient effectmay not be obtained occasionally depending on the process conditions.Accordingly, it is difficult to stably maintain the excellentcharacteristics in the reaction chamber.

The present invention has been achieved in view of the foregoingcircumstances and a primary object thereof is to enable a transistoradapted to suppress characteristic degradation resulting from fluorinecontained in the thin film composing the transistor to provide excellentreliability even when it is continuously driven for a long period oftime at a relatively high temperature.

To attain the object, the present invention focuses attention on thegate insulating film of the transistor and reduces the concentration offluorine contained in the gate insulating film to 1×10²⁰ atoms/cm³ orless, preferably to 1×10¹⁹ atoms/cm³ or less.

When the gate insulating film is deposited by using a CVD apparatus, thesurface of an electrode is composed of a non-porous layer in thereaction chamber of the CVD apparatus as specific means for thusimplementing a reduction in the concentration of the contained fluorine.

As a result, carriers trapped by fluorine in the surface of the gateinsulating film in contact with the semiconductor film are reduced sothat the ON-state current characteristic of the transistor is improved.At the same time, fluorine ions in the gate insulating film are reducedso that the threshold characteristic of the transistor is improved. Inaddition, even when the transistor is continuously driven for a longperiod of time at a relatively high temperature, excellent reliabilityis obtainable.

When the gate insulating film is deposited by using a CVD apparatus, theroot cause of the remaining of fluorine in the porous layer (e.g.,formed by an anodic oxidation process for forming a protective film) asthe surface of the electrode in the reaction chamber thereof can beremoved. Compared with the case where the process of removing residualfluorine by using a hydrogen plasma is performed, the production of afaulty transistor resulting from insufficient removal of fluorine due tovariations in process conditions and the resultant reduction in yieldcan be suppressed.

In the structure described above, the transistor is preferably of afield-effect type. The gate insulating film is preferably an amorphoussilicon nitride film. The gate insulating film is preferably depositedby a CVD method. The transistor described above is suitable for use as aswitching element for a pixel electrode portion in a liquid crystaldisplay device.

By adjusting the concentration of fluorine contained in the gateinsulating film of a transistor to 1×10²⁰ atoms/cm³ or less, preferablyto 1×10¹⁹ atoms/cm³ or less, the present invention not only allowsexcellent initial characteristics to be obtained but also allows animprovement in reliability even when a transistor is continuously drivenfor a long period of time at a relative high temperature, as in the casewhere it is used in a liquid crystal display device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view schematically showing an overallstructure of a field-effect thin-film transistor according to anembodiment of the present invention;

FIG. 2 is a schematic view showing an overall structure of a CVDapparatus used to deposit a gate insulating film;

FIG. 3 is a cross-sectional view schematically showing a structure of asurface of an anode in the reaction chamber of the CVD apparatus;

FIG. 4 is a view schematically showing a structure of a surface of ananode in the reaction chamber of a conventional CVD apparatus, whichcorresponds to FIG. 3;

FIG. 5 is a characteristic view showing the relationship between theconcentration of fluorine contained in a gate insulating film and thethreshold voltage of a transistor; and

FIG. 6 is a characteristic view showing the relationship between theconcentration of fluorine contained in the gate insulating film and theON-state current characteristic of the transistor.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to the drawings, the embodiment of the present invention willbe described herein below. It is to be noted that the present inventionis not limited to the following embodiment and can be designedappropriately as necessary.

FIG. 1 schematically shows a cross section of a field-effect thin-filmtransistor according to the present embodiment. The transistor is usedas, e.g., a switching element for a pixel electrode portion in a liquidcrystal display device.

The foregoing transistor comprises an insulating substrate (1) made of,e.g., glass or the like. A gate electrode (2) made of Ta, Al, Mo, or thelike is formed on the substrate (1). A gate insulating film (3) composedof, e.g., an amorphous silicon nitride film with a thickness of, e.g.,4000 Å is formed on the gate electrode (2) to cover substantially theentire surface of the substrate (1). An amorphous silicon semiconductorfilm (4) as a semiconductor film with a thickness of, e.g., 2000 Å isformed on the gate insulating film (3) with the portion thereofcorresponding to the gate electrode (2) being disposed at the center. Apair of n⁺ amorphous silicon semiconductor films (5) each having athickness of, e.g., 500 Å and doped with phosphorus are formed on thetwo regions of the amorphous silicon semiconductor film (4) with thegate electrode (2) interposed therebetween to serve as semiconductorfilms other than the amorphous silicon semiconductor film (4). A sourceelectrode (6) and a drain electrode (7) each made of Ti, Mo, Al, or thelike are formed over the individual n⁺ amorphous silicon semiconductorfilms (5) and the respective portions of the gate insulating film (3)continued thereto.

A description will be given next to the process steps of fabricating thetransistor thus constituted. First, the gate electrode (2) is formed onthe substrate (1) through deposition and patterning. Then, a diodeparallel-plate plasma-enhanced CVD apparatus is used and, after thecleaning of the reaction chamber thereof using NF₃ gas, the gateinsulating film (3) is deposited. Thereafter, a first semiconductor filmfor obtaining the amorphous silicon semiconductor film (4) and a secondsemiconductor film for obtaining the n⁺ amorphous silicon semiconductorfilms (5) are deposited individually. Then, a multilayer film comprisingthe first and second semiconductor films is patterned into an islandconfiguration to form the amorphous silicon semiconductor film (4)first.

Further, the source electrode (6) and the drain electrode (7) are formedover the specified portions of the gate insulating film (3) and themultilayer film through deposition and patterning. Then, by using thepattern of the source electrode (6) and the drain electrode (7), etchingfor dividing the second semiconductor film is performed with respectthereto to form the n⁺ amorphous silicon semiconductor films (5). By theprocess described above, the field-effect thin-film transistor iscompleted.

A description will be given herein below to the reaction chamber of theplasma CVD apparatus mentioned above with reference to FIG. 2. In thereaction chamber (50), an anode (52) made of aluminum and having a largenumber of gas supply holes (51) is disposed. Unlike in the conventionalcase, an anodic oxidation process for forming a protective film has notbeen performed with respect to the surface of the anode (52).Accordingly, as schematically shown in enlarged relation in FIG. 3, thesurface of the anode (52) is composed of an aluminum layer (70) as anon-porous film which is exposed in an unprocessed state.

Specifically, in the conventional case, an anodic oxidation protectivefilm (61) made of alumite is formed on the surface of the aluminum layer(70) by the anodic oxidation process for forming the protective film, asschematically shown in FIG. 4. Because the anodic oxidation protectivefilm (61) is porous, fluorine is likely to be trapped by the insides offine holes therein, which causes a large amount of fluorine to remain inthe reaction chamber (51) after cleaning. By contrast, the presentembodiment has eliminated the root cause described above by forming thealuminum layer (70) as the surface of the anode (52), i.e., by notforming the anodic oxidation protective film (61) as formedconventionally on the surface of the aluminum layer (70). As means forimplementing the non-porous surface, a new non-porous layer may also beformed on the surface of the aluminum layer (70).

For comparison, by using a conventional CVD apparatus in which thesurface of the anode (52) is composed of the anodic oxidation protectivefilm (61), cleaning of the inside of the reaction chamber (50) wasperformed and then a hydrogen plasma process was performed continuouslyfor 60 seconds under conditions such that the output of aradio-frequency power source (RF power source) was 1000 W and the flowrate of gas was 3 L/min (at 1.013×10⁵ Pa and 0° C.). As a result, theconcentration of fluorine contained in the gate insulating film (3) was3×10²⁰ atoms/cm³. By contrast, in the present embodiment, values of7×10¹⁸ to 1×10¹⁹ atoms/cm³ were steadily obtained under the sameconditions.

Although the foregoing embodiment has described the case where the twosemiconductor layers, which are the amorphous silicon semiconductor film(4) and the n⁺ amorphous silicon semiconductor film (5), are used, thepresent invention is also applicable to a transistor using asingle-layer semiconductor film.

Although the foregoing embodiment has described the case where the gateinsulating film (3) is an amorphous silicon nitride film, the presentinvention is also applicable to a transistor in which the gateinsulating film (3) is a film other than the amorphous silicon nitridefilm such as, e.g., an amorphous silicon dioxide film, an amorphousaluminum oxide film, or the like.

Although the foregoing embodiment has used the anode (52) having thesurface thereof composed of the non-porous layer in the reaction chamber(50) to reduce the concentration of fluorine contained in the gateinsulating film (3) when the gate insulating film (3) of the transistoris deposited by using the CVD apparatus, means for reducing theconcentration of the contained fluorine is not particularly limited.Another means can be used as appropriate.

Although the foregoing embodiment has described the case where a plasmaCVD method is used to deposit the gate insulating film (3) of thetransistor, the present invention is also applicable to the case using asputtering method such as, e.g., RF sputtering, ECR sputtering, orreactive sputtering. In the case of using a CVD method also, the presentinvention is applicable to a transistor in which the gate insulatingfilm (3) is deposited by a method other than plasma CVD method such as athermal CVD method or an optical CVD method.

Although the foregoing embodiment has described the case where thetransistor is used for an application as a switching element for a pixelelectrode portion in a liquid crystal display device, the presentinvention is also applicable to a transistor used for anotherapplication.

EXPERIMENTAL EXAMPLES

A description will be given herein below to an experiment performed toexamine the concentration [Unit: atoms/cm³] of fluorine contained in thegate insulating film of a transistor and the respective initialcharacteristics of the threshold (Vth [Unit: V]) and ON-state current(Ion [Unit: nA]) thereof. As for the concentration of the containedfluorine, it was determined by depositing an amorphous silicon nitridefilm under the same conditions as for a gate insulating film on asilicon wafer and measuring the concentration of fluorine contained inthe amorphous silicon nitride film by secondary ion mass spectrometry(SIMS). As the transistor, a transistor in which the ratio between thechannel width W and the channel length L was W/L=4 was used.

The characteristic view of FIG. 5 shows the initial characteristicbetween the concentration of fluorine contained in the gate insulatingfilm and the threshold voltage of the transistor. The characteristicview of FIG. 6 shows the initial characteristic between theconcentration of fluorine contained in the gate insulating film and theON-state current characteristic of the transistor.

From the drawings, it will be understood that the initial transistorcharacteristics are excellent provided that the concentration of thecontained fluorine is 1×10²⁰ atoms/cm³ or less, preferably 1×10¹⁹atoms/cm³ or less.

A description will be given next to an experiment performed to examinethe relationship between the concentration of fluorine contained in thegate insulating film and the reliability of the transistor during thelong-period driving thereof at a high temperature.

In the conventional case (see Patent Gazette 2), the reliability wasevaluated under conditions such that each of the source and drain wasgrounded, the gate DC voltage was 30 V, and the drive time was 10minutes in an environment where the temperature was 25.0±3.0° C.However, when a field-effect thin-film transistor is applied to, e.g., aliquid crystal display device, the guarantee of the operation at ahigher temperature for a longer period of time is requested so that theevaluation of the reliability was performed herein under conditions suchthat each of the source/drain electrodes was grounded, the gate DCvoltage was 15 V, and the drive time was 500 hours in an environmentwhere the temperature was 80.0±3.0° C. As an index for judging thereliability, an amount of shift ΔVth [Unit: V] as a value obtained bysubtracting the threshold voltage after a reliability evaluation testfrom the initial threshold voltage of the transistor was evaluated.

As a result, in the transistor in which the concentration of thecontained fluorine was 2.7×10²⁰ atoms/cm³, the amount of shift wasΔVth=5.0 V, while ΔVth=3.0 V was measured in the transistor in which theconcentration of the contained fluorine was 1.0×10²⁰ atoms/cm³. Thisproved that the reliability under high-temperature and long-periodconditions improved more greatly as the concentration of fluorinecontained in the gate insulating film was lower.

The present invention can be used for a typical common transistor and isparticularly preferable for a transistor which is continuously drivenfor a long period of time at a relatively high temperature, such as atransistor used as a switching element for a pixel electrode portion ina liquid crystal display device.

1. A CVD apparatus used to deposit a gate insulating film of atransistor including a source electrode and a drain electrode arrangedopposite to each other, a semiconductor film including at least onelayer disposed between the source electrode and the drain electrode, agate electrode disposed adjacent to the semiconductor film, and the gateinsulating film disposed between the gate electrode and each of thesource electrode, the drain electrode, and the semiconductor film, theCVD apparatus comprising: an electrode having a plurality of gas supplyholes and disposed in a reaction chamber, the electrode including asurface that is composed of a non-porous layer; wherein the transistoris disposed in the reaction chamber, the transistor is a field-effecttransistor in which the gate insulating film and the semiconductor filmare formed in that order, and the semiconductor film is disposed on thegate insulating film; and the electrode of the CVD apparatus is adaptedto form the gate insulating film such that the gate insulating film ismade of an amorphous silicon nitride film and has a concentration offluorine in a range of about 7×10¹⁸ atoms/cm³ to about 1×10²⁰ atoms/cm³.2. A transistor comprising the gate insulating film formed by the CVDapparatus according to claim 1.